In liquid transportation through a pipeline to a destination, in general, it is often a usual practice to transport different liquids simultaneously through a single pipeline with a view to effectively utilizing the pipeline. It is necessary in this case to accurately detect the boundary between different liquids flowing in the pipeline from outside the pipeline at the destination and to separate a liquid from the others by switching over the route of the pipeline at said boundary.
There are several methods available for detecting from outside the pipeline the boundary between different liquids flowing in a pipeline. Among others, a method for detecting said boundary based on the sing-around method is known, which employs the fact that the sound velocity varies with the change in the natural physical properties of a medium such as the density and the bulk modulus.
The sing-around method, which permits continuous measurement of the velocity of an ultrasonic wave, is based on the following principle: fixing a transmitting probe and a receiving probe at a certain distance; emitting an ultrasonic pulse from the transmitting probe into a liquid in response to a voltage signal from a pulse oscillator; receiving said ultrasonic pulse by the receiving probe; converting said received pulse into a voltage signal; subjecting said voltage signal to an amplification and a wave-forming; starting up again the pulse oscillator with the thus processed voltage signal to emit an ultrasonic pulse again from the transmitting probe into the liquid; thus repeatedly emitting an ultrasonic pulse from the transmitting probe within a closed loop; counting the frequency of repetition of the ultrasonic pulse (hereinafter referred to as "sing-around frequency") on these repeated emissions with a frequency counter; and calculating the sound velocity through the liquid in accordance with the following equation: EQU 1/f = l/c
Where,
l: distance between transmitting and receiving probes, PA1 c: sound velocity through the liquid, and PA1 f: sing-around frequency.
The time lag in the electric circuit, which should be taken into account in the above-mentioned equation, is omitted here for simplification of description.
FIG. 1 is a drawing schematically illustrating the conventional apparatus for detecting the boundary between different liquids flowing in a pipeline, based on the above-mentioned sing-around method. For the purpose of preventing multiple reflection of an ultrasonic pulse, as shown in this drawing, a transmitting probe 2 and a receiving probe 3 are provided on the outer surface of a pipeline 4 at an angle therewith, so that an ultrasonic pulse may be emitted at said angle with the flow direction of the liquid in the pipeline 4 (indicated by an arrow in the drawing). An ultrasonic pulse is repeatedly emitted from the transmitting probe 2 into the liquid as described above. The sing-around frequency is converted into a voltage output by a frequency/voltage converter 5, and said voltage output is recorded in a recorder 6. In FIG. 1, 1 indicates a sing-around circuit. A change in the density of liquid flowing in the pipeline 4, that is, the passage of a boundary between different liquids across a given point, leads to a change in the sing-around frequency, and hence to a change in the voltage output. It is thus possible to detect the boundary between different liquids from outside the pipe 4.
In the above-mentioned conventional sing-around circuit which employs an external forced synchronizing astable multivibrator (hereafter referred to as "astable multivibrator") as the pulse oscillator, the frequency of a transmission pulse of the astable multivibrator is synchronized with the frequency of a signal from the receiving probe, and the resulting synchronized transmission pulse is used as a transmission signal.
In a free state in which an external forced synchronizing signal (hereafter referred to as "synchronizing signal") is not given as a trigger signal, the astable multivibrator emits pulses in succession with a preset frequency and pulse width as shown in FIG. 2 (a). A synchronizing signal, if supplied during the on-state of the transmission pulse, causes no response of said transmission pulse to said synchronizing signal, whereas, during the off-state of said transmission pulse, said transmission pulse responds to a synchronizing signal, if it is supplied.
More specifically, if a synchronizing signal having a frequency slightly higher than that of the transmission pulse, as shown in FIG. 2 (b) or FIG. 2 (d), is supplied during the off-state of the transmission pulse of the astable multivibrator, the frequency of said transmission pulse is locked to the frequency of said synchronizing signal and synchronized therewith. The astable multivibrator emits accordingly a pulse with a frequency equal to that of said synchronizing signal and with its own pulse width, as shown in FIG. 2 (c) or FIG. 2 (e). On the contrary, when a synchronizing signal as shown in FIG. 2 (f) is supplied during the on-state of the transmission pulse of the astable multivibrator, said transmission pulse shows no response to said synchronizing signal, and hence, the astable multivibrator emits a pulse, as shown in FIG. 2 (g), with the same preset frequency and pulse width as those shown in FIG. 2 (a).
The boundary between different liquids flowing in a pipe has conventionally been detected with the use of the above-mentioned characteristic of the astable multivibrator. This conventional method has more specifically comprised setting the frequency of transmission pulse of an astable multivibrator at a value slightly lower than the sing-around frequency corresponding to a liquid flowing in a pipe giving the lowest sound velocity, and synchronizing the frequency of transmission pulse of said astable multivibrator with the sing-around frequency, i.e., with the frequency of a signal from a receiving probe, thereby using the thus synchronized transmission pulse as the transmission signal. An erroneous detection caused by noise has been prevented by masking received noise waves such as ultrasonic waves propagating along a pipe wall through adjustment of the pulse width of the transmission pulse of said astable multivibrator to a slightly wider width.
However, when employing an astable multivibrator in a sing-around circuit, it is inevitable that the following difficulties are encountered:
(1) When alteration of the kinds of liquids flowing in a pipe has caused a change in the sing-around frequency, the preset frequency of a transmission pulse of the astable multivibrator may exceed the limit of synchronization thereof with said sing-around frequency, i.e., with the frequency of a pulse from the receiving probe, depending upon the extent of the preset value of the frequency of the transmission pulse of said astable multivibrator itself, thus giving rise to the fear of impossibility of synchronization.
(2) When a ultrasonic pulse from the transmitting probe has not reached the receiving probe in a normal manner under the effect of air bubbles or foreign matters entangled into liquids flowing in a pipe, an erroneous detection may be caused unless instruments are carefully watched.
(3) The frequency and the pulse width of the transmission pulse of the astable multivibrator itself cannot be independently adjusted. It is therefore practically very difficult to adjust said pulse width to a larger desired value with a view to masking noise such as ultrasonic waves propagating along a pipe wall.
(4) When the astable multivibrator is used for a pipe of a different diameter, it is necessary to adjust the frequency and the pulse width of the transmission pulse thereof to values suitable for the diameter of the new pipe. It is however very difficult to make such an adjustment as mentioned above, and in particular, it is impossible to make such a site adjustment.
For these reasons, there has been a demand for an apparatus for detecting a change in paramenters of a liquid flowing in a pipe based on the sing-around method, which is free from the troubles mentioned above, but such an apparatus has not as yet been proposed.